High-frequency heating device



' Feb. 25, 1958 w. KUHLMANN 2,824,940

HIGH-FREQUENCY HEATING DEVICE Filed July 15, 1954 &\ \1

INVENTOR WALTER K U HLMANN AGENT 2,s24,94o r HIGH-FREQUENCY HEATING DEVICE Walter Kuhlmann, Hamburg,

mesne assignments, to North pany, Inc., New York, N. ware Germany, assignor, by American Philips Com- Y., a corporation of Dela- The present invention relates to high-frequency heating. More particularly, the invention relates to a screened space for capacitative high-frequency heating of workpieces, for example for gluing plywood sheets, bodies of different layers of wood of curved form and the like. It is known that such work pieces are subjected to highfrequency heating in a screened space in order to prevent a radiation of the required very high energy, which may disturb radio reception in the neighbourhood.

With certain oscillator circuits used for this purpose the resonant circuit of the oscillator includes the work piece connected in series with the inductor of this circuit by connecting it between the electrodes ofa capacitor connected to the coil. In general, this capacitor is connected to the resonant circuit through a concentric high- I frequency conductor.

It has now been found that the capacity of this capacitor, which exhibits in disturbs the resonance frequency. According to the present invention this disadvantage is avoided and in accordance with the present invention, the wall of the screened space connected to the outer conductor of the concentric high-frequency supply cable is connected through a Litzen-wire to one electrode to be arranged on the work piece, while the other electrode, connected to the inner conductor of the cable, is arranged in the space in a manner such that its capacity relative to the wall of the space is at least equal to, preferably however many times higher than the capacity between the electrodes themselves. The effect of such an arrangement is that the capacity of the plate capacitor fluctuating to a high extent with the work piece to be treated has connected with it in parallel an additional capacitor having a nonvariable, comparatively high value, so that fluctuations have comparatively less effect.

The space may be constructed at least in part from resilient, discontinuous material, for example a wire net which is compressed together with the electrode connected to the inner conductor with the interposition of a dielectric having low high-frequency losses. The use of a wire net or of a similar material has the advantage that during the treatment the work piece can be observed and that any gases or vapors developed can escape. The space itself, if freedom from interference is required, may be provided with a double wall in known manner, so that a double screening is obtained.

If a work piece is to be heated in the space requiring a gauge, for example the seat of a chair made from various layers of Wood, or the like, it is of advantage to construct the space in a manner such that a gauge required to form the work piece lies at least partly outside the space and the resilient wall of the space is temporarily deformed by the gauge. Thus even complicated shapes may be manufactured without the need for further means, and it is possible to exert the required pressure on the work piece without the use of through-connections in the wall of the space which give rise to structural difficulties.

practice comparatively large fluctuations,

United States Patent i 2,824,940 Patented Feb. 25, 1958 In order that the invention may be clearly understood and readily carried into effect, reference may be had to the accompanying drawing, wherein:

Fig. l is a schematic diagram in sectional view of an embodiment of the device of the present invention; and

Fig. 2 is a schematic diagram in sectional view of a modification 0f the embodiment of Fig. 1.

According to Fig. 1 the concentric cable of the highfrequency generator is designated by 1; the cable is connected to an elongated space, of which part 2 is made of sheet iron and the upper part 3 of a wire net. At the right-hand end of the space provision is made of a window 4 of angular metal; the window can be closed by means of a flap 5 and serves for introducing or removing the work piece.

The space is provided with two plate-shaped electrodes 6 and 7, of which the first is connected through a flexible Litzen-wire 8 to the wire net 3, which is connected in turn to the grounded screening sheath of the concentric cable 1. The inner conductor of the cable 1 is connected directly to the electrode 7. The electrode 7 is supported by two stay members 9 and 9, which are made from a dielectric material suitable for high-frequencies and which have a size such that a suitable capacity between the electrode 7 and the space wall is produced. A piece 10 of different layers of wood, which are to be joined by means of suitable glue by high-frequency heating, is placed between the electrodes 6 and-7. Provision is made of suitable pressure means comprising a screw spindle 11, which exerts pressure, through a suitable intertnediate layer, on the electrode 6 and hence 'on the work piece 10.

In proportion to the dimensions of the work piece to be treated between the electrodes 6 and 7 a higher or a less high capacity is produced between these two electrodes, while in the device shown the value can fluctuate only Within narrow limits, since the capacity determined by the dielectrics 9 and 9' is in parallel with the capacitor formed by the electrodes 6 and 7. Thus only a slight reaction is exerted on the oscillator.

A modification of the embodiment of Fig. 1, for joining curved plywood sheets, is shown in Fig. 2. The space is constituted in this case by a wire net 3 which surrounds the work piece and which can be opened at the end remote from the concentric conductor 1 in order to render the interior accessible. After the required parts have been placed therein, the space is closed; in this case simply by winding up the open end of the wire net 3, for example in the manner shown. Inside the space, provision is made of the electrodes 6 and 7, which are connected to the metal wall of the space and to the inner conductor of the concentric cable 1, respectively. The dielectric 9, having low high-frequency losses is made'of a resilient material, for example synthetic resin.

It is assumed that a work piece 10 of the curved shape shown is to be manufactured; this requires gauges 12 and 12', of which one is positioned outside the space, and the other is positioned inside the space. This embodiment is given only by way of example. The gauge part 12' may also be positioned outside the space. When the screw spindle 11 is tightened, the pressure is transferred to the gauge parts 12 and 12, to the work piece 10 and to the insulating intermediate layer 9, and said work piece is shaped in the desired form shown, determined by the shape of the gauge. The high-frequency heating may be carried out in a conventional manner; the dielectric layer 9 produces the effective paralled capacity.

While the invention has been described by means of specific examples and in specific embodiments, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

itance-between"said-"one-electrodeand said enclosure is substantially greater than the electrical icapa'citance" between said pair-of*electrodes, said last-mentionedcapacitancebein'g-in'parallelwith ,said first-mentioned capacitance.

' 2. A high-frequency-heating device-as claimed in'claim 1, wherein-said last-mentioned means comprises dielectric material having lowhigh=frequency losses and interposed between said one electrode and said electrically conductive material.

3. A high-frequency heating device, as claimed in claim -1,-furthercomprising at "least one gauge "for" shaping a body'positioned between said pairof electrodes, said'gauge beingpositioned "at least partly outside said enclosure,

-said'electricallyconductive material being shaped-by said gauge upon engagement therewith.

4. -A highfre'quency heating device comprising a metallic portion and a-wire screenforming an enclosurewith saidmetallic-portion,-said-portion"and said screen being electrically conductive,--a-coaxial"cable, a pair of spaced heating electrodes positioned'in said enclosure, means for supplying energy from said cableto said'pair of electrodes and tosaid-portion,=and-dielectric material'having low high-frequency losses interposed "between 'one' of said pair of=-electrodes andsaid portion ina'manner whereby the "electricalcapacitance-between said one-electrode and said itance-between-saidpair of electrodes, said last-mentioned capacitance being in parallel with said first-mentioned capacitance.

5. A high-frequencyheating device for heating a body positioned between a pair of electrodes comprising an electrically conductive wirescreen'formingvanenclosure, a coaxial cable, a pair of spaced heating electrodes positioned irr sa'id enclosure, meansfor supplying energy from said: cableto said pair'ofielectrodesand' to said screen, a gauge for shaping said body, ariddielectric material having 10w high-frequency losses interposed between one of said pair tofzelectrodesaand a. portion of saidscreen in a manner whereby the electrical capacitanceLbetween said one electrode and said portion is substantially greater than the electrical capacitance;betweenzsaidpair of electrodes, said portion being interposed between said dielectric material and saidagauge andabeing shaped. by the,said;gauge, said last-mentioned capacitance being in parallel ,withpsaid first-mentioned capacitance.

.R'e'ferences' Cited in lthe file of thispatent UNITED STATESPAT-ENTS 1,962,796 'Wappler J11r1e312f1934 2,073,880 Robinson Mar. 16, 1937 2,203,517 'Beggs June 4, 1940 2,467,782 Schuman Apr. 19', 1949 2,572,226 Walstrom Oct. 23, 1951 2,71-13468 "Ellsworth "June 21, 1955 "2,732,473 "Ellsworth Jan. 24, 1956 V :FOREIGN P-ATENTS 556,292 7 Great Britain. Sept. 28, T1943 

